Proton-exchange membranes (PEM) are useful in many applications, including fuel cells such as hydrogen fuel cells (H2—FC) and direct methanol fuel cells (DMFC). A PEM is typically a semipermeable membrane generally made from ionomers that conducts protons but is impermeable to gasses such as oxygen or hydrogen. In a PEM fuel cell (PEMFC), the PEM severs two functions: separating the reactants, and transporting protons across the membrane. A PEM functions as a polymer electrolyte membrane. A common, and commercially available, PEM material is Nafion™, manufactured by DuPont™. A PEM can be primarily characterized by its proton conductivity, methanol permeability, mechanical strength, and chemical stability. The proton conductivity in a PEM can be significantly affected by the water content in the PEM—loss of water can reduce the proton conductivity.
Conventional PEMs have some drawbacks. For example, in a Nafion membrane, water loss becomes significant at temperatures above 80° C. and the permeability of methanol is high. As a result, a Nafion membrane is not suitable for use in H2—FCs at elevated temperatures, due to reduced proton conductivity caused by dehydration; and is not suitable for use in DMFCs, due to extensive permeation of methanol.
It has been reported that Nafion membranes doped with silica nanoparticles can improve their performance. Such composite membranes are reported to exhibit improved swelling behavior, thermal stability and mechanical properties. However, silica nanoparticles tend to aggregate within the membrane and it is difficult to distribute the silica particles uniformly in the membrane. Further, test data shows that the proton conductivity of the composite membrane remains low, even lower than that of pure Nafion membrane at temperatures from 50 to 80° C.
Accordingly, there is a need for proton-exchange composites that have improved properties such as proton-conductivity, thermal stability, and reduced permeability of water and methanol.